Rotary hearth furnace



Aug. 23, 1938.

K. R. GOHRE ET AL ROTARY HEARTH FURNACE Filed April l3, 1937 5Sheets-Shet 1 Inventors.-

KlLl/ZR. G'b'bre Jakob Sc]: M216 Paul Jpez'c/zerf Atlbrneg .23.'1938.MGQHREETAL 2,1 7 127 ROTARY HEAR'I'H FURNACE Filed April 13. 1937 5Sheets-Sheet F1912 A flax-neg Aug. 23, R Q AL 2,127,727

ROTARY HEARTH FURNACE:

Filed April 13, 1937 5 Sheets-Sheet 4 PM! 5 eicherl B, OVWM Attorney Inrenters.-

Patented Aug. 23, 1938 UNITED STAT ES PATENT OFFICE ROTARY nnaa'rnFURNACE Application April 13, 1937, Serial No. 186,576

In Germany September '1, 1935 18 Claims.

1 The present invention relates to rotary hearth furnaces, and, moreparticularly, to rotary hearth furnaces for producing lead from sulphideores by means of the roasting reaction process.

As those skilled in the art know, furnaces having a rotatable hearth ofannular form provided the best results in treating sulphide ores oflead. These rotary hearth furnaces were generally provided with astationary hood which extended into the close proximity of the exteriorrim of the hearth. The hood was provided with one or with a plurality ofworking openings or doors for the treating of the charge and chargingand stirring apparatus were provided at both This arrangement providedsuch conditions that even in the case of relatively small air leakage,the gas velocity in the openings of ,the hood and in the gap between thehood and the rim of the hearth was sufficient to completely prevent thegases or fumes formed during the reaction to, escape to The rotaryhearth furnace provided a very substantial progress in the production oflead from sulphide ores in that it eliminated the presence of poisonousfumes in the proximity of thefurnace, increased the output and reducedthe cost of production. It has been found, however, that even in rotaryhearth furnaces substantial losses occurred through the formation offlying dust and of lead fumes. Particularly in proximity to the rabblesemployed for the purpose of breaking up the charge, the air blastcontinuously emitted through the nozzles readily picked up smallparticles of the charge and cause the production of clouds of ying dustand lead fumes. Likewise, the rabbles had to be operated at a relativelyhigh speed in order to break up the charge and to remove incrustationson the hearth in the proximity of the nozzles and the necessary quickmovements of the rabbles had the detrimental effect of sprayingsubstantial amounts of lead and to produce additional amounts of flyingdust and lead Further amounts of flying dust and lead fumes have beencaused by the irregular circula- 1 tion and currents of the air enteringthrough the working doors and through the gap between the of thefurnace. Of course, the losses and' other inconveniences caused throughflying dust and lead fumes reduced the eiiiciency of rotary hearthfurnaces and prevented to fully realize all of the advantages connectedwith the employment of the sides of these working doors.

the exterior.

fumes.

hood and the exterior rim rotary hearth principle.

We have discovered that all of the above difficulties may be eliminatedin a remarkably simple manner.

It is an object of the present invention to provide an improved rotaryhearth furnace .which is substantially free from the disadvantages andinconveniences connected with conventional hearth furnaces.

Another object of the present invention is to provide an improved rotaryhearth furnace particularly for the production of lead from sulphideores by means of the roasting reaction process which substantiallyeliminates and pre vents the formation of flying dust and of lead fumesand the losses connected therewith.

A further object of the invention is to provide a rotary hearth furnacefor smelting lead 'ores involving an improved control of the air blastthrough the charge. 1

It is also within the contemplation of the in- .vention to provide anovel and improved rabbling and stirring apparatus for breaking up thecharge and for the removal of incrustations substantially withoutcausing the formation of flying dust and of lead fumes. 1

Moreover, the invention also contemplates an improved arrangement of thehood and its cooperating parts with respect to a rotatable hearth ofannular shape in order to prevent the formation of irregular and eddycurrents of air within the hood and to eliminate the production oflocalized overheating of the charge, formation of flying dust andof leadfumes.

The invention likewise contemplates an improved rotary hearth furnacewhich is simple in construction and which may be built and operated on apractical and industrial scale at a low cost.

Other and further objects and advantages of the invention will becomeapparent from, the following description taken in conjunction withrotary hearth furnace embodying the principles of the present invention;

Fig. 2 depicts a vertical sectional view of a modified embodiment of theinvention, taken on the line 2-2 of Fig. 3;

Fig. 3 shows a section taken on line 3-3 of Fig. 4 is a verticalsectional view of a further modified embodiment of the invention takenon line 4-4 01 Fig. 5;

Fig. 5 illustrates a top elevational view of the rotary hearth furnaceshown in Fig. 4;

Fig. '6 shows a fragmentary top elevational the accompanying drawings,in which: i

Fig. 1 illustrates a vertical sectional view of a view having parts insectionof a portion of the furnace shown in Fig. 1 and illustrating thearrangement for controlling the air blast to the charge and for cleaningthenozzles;

Fig. 7 is a similar view of the rabbling mechanism.-

Fig. 8 illustrates a vertical sectional view of a further modifiedembodiment of the invention. taken on .line 8--8 of Fig. 9; and

Fig. 9 depicts a top elevational view of the hearth shown in Fig. 8.

Broadly stated, according to the principles of the present invention,the formation of flying dust and of lead fumes is reduced to a minimumor is completely eliminated by providing intermit- 'tent operation ofthe nozzles employed for the the time they are in immediate proximity ofthe rabbling apparatus. This removes one of the principal causes offlying dust and lead fumes.

In addition to this, the rabbling apparatus is so constructed andarranged that the breaking up and the rabbling of the charge is carriedout cautiously and at a relatively low speed without causing spraying ofthe lead. Moreover, the charging apparatus and the stationary hoodlocated above the hearth with its gas conduits are so constructed thatthe production of dust in the newly added charge and the volatilizationof lead are greatly reduced and that the small amounts of flying dustaccidentally produced are deposited already within the hood. In thisman.. ner, the principles of the present invention make it possible toeliminate all of the causes for the formation of flying dust and for theoccurrence of volatilization of lead.

In the elimination of flying dust and of lead fumes, the proper controlof the nozzles is of the greatest importance. In prior rotary hearthfurnaces, the nozzles have been generally. freed from the coveringcharge materials in the portions of the hearth where the rabbles arebreaking up the charge so that the air blast from the correspondingnozzle was blowing over the charge. Of course, this circumstance greatlyincreased the production of flying dust and of lead fumes. According tothe principles of the present invention, this most important cause ofthe formation of flying dust is eliminated by shutting off the nozzleswhich are in immediate proximity of the rabbling apparatus. Preferably,the nozzles are shut off by means of an apparatus consisting of aplurality of rods which are rotatably mounted on a common verticalshaft, for example, in the form of a star arranged in the plane of thenozzles. This nozzle controlling apparatus is so arranged within thewindbox that its axis of rotation is substantially in a vertical planepassing through the axis of rotation of the hearth and through therabble of the rabbling apparatus. The rods have such dimensions that theone which happens to be in proximity to the rabble will just aboutprotrude through the nozzle which is in the corresponding position. The.distance of the free end of the rods from each other corresponds to thedistance between the nozzles so that rotation of the hearth will causesuccessive engagement of the rods-with the nozzles of the hearth andwill set up rotation of the rods around their common axis. As soon asone of the rods engages a nozzle, the passage of air through said nozzleis greatly reduced or completely prevented, so that in the region of therabbling apparatus no air blast is blown into the hearth. A furtherimportant advantage of this arrangement is that the engagement anddisengagement of the rods with the nozzles automatically clean thenozzles, in that charge particles which entered the nozzles andincrustations formed within the nozzles are removed by the rods and arereturned to the hearth. Thus, manual cleaning of the nozzles, which hasbeen absolutely necessary in prior rotary hearth furnaces, is completelyeliminated and the servicing and supervision of the furnace are greatlyfacilitated.

In order to further reduce the formation of lead fumes in the proximityof the rabbling apparatus, the invention provides an improved rabblingapparatus which permits relatively slow operation of the rabbles. Asthose skilled in the art know, the object of the rabbles in the hearthfurnaces of the type contemplated is not merely to break up the chargebut also to remove the incrustations formed on the back wall of thehearth in the proximity of the nozzles. This object can be accomplishedonly if the rabble sweeps over all of the surface of the back wall ofthe hearth during the time the portion of the rotating hearth is withinthe reach of the rabble. Generally speaking, in the operation of rotaryhearth'furnaces it is highly' desirable to maintain relatively fastrotation of the hearth in order to obtain a high output. Of course,under these operating conditions it is necessary to make the rabblerelatively wide and to operate. the same at a high speed, becauseotherwise it would be impossible to obtain overlapping of the individualrabble strokes on the surface of'the hearth. The great width and thefast operation of the rabble, however, has the disadvantage thatsubstantial amounts of lead are sprayed about and that simultaneouslywith the breaking up of the charge very great quantities of dust areformed.

In order to eliminate also this cause of the formation of dust andndifumes, the presentinvention provides a plurality of rabbles, generallytwo, arranged in close proximity of each other. Preferably, the rabblesarerelatively narrow and are operated alternately. This makes itpossible .to operate the rabbles at a relatively lowspeed so thatsplashing -orspraying of the lead is avoided and a cautious butcompletely satisfactory breaking up of the charge is obtained. It ispreferred to provide the rabbles at a slight distance from each otherand to operate them by means of individual eccentrics removablyconnected to a common drive. This makes it possible to individuallydisconnect each rabble and to remove the same from the charge for thepurposes of inspection, repair, or replacement.

Thus, the furnace embodying the principles of the invention successfullyeliminates the principal causes of the formation of dust and of fumesand the only remaining possibility of the formation of dust .and offumes is in the irregular movement or streaming of the air enteringthrough the working doors and through the gap between the hood and theexterior rim of the furnace, the so-called'air leakage, or false draft.Damming up of this air and the formation of eddy currents therein maycause picking up of increasedpuantities of dust from the chargedropped'onto the hearth. Moreover, it can easily happen that certainportions of the hearth are not suiliciently exposed to theeilect of thisdraft which may cause localized overheating of the charge andconsequently volatiiization of subto strong currents of gas.

Thus, for example, the fresh charge material consisting of wellmoistened ore mixed in a suitable proportion with coalmay be fed to anannular charging plate provided within or without the hood and rotatingwith the hearth. This charging plate is only at a slightly higher levelthan the charge material on the hearth so that the height of fall fromthe charging plate to the hearth is very low and is incapable of caus--ing the production of flying dust. In addition to this, premature drying'out of the charge ma-- terial in the charging apparatus is prevented byproviding the discharge end of the charging bin size where the charge isfed simultaneously to the hearth at a plurality of points. It is ofadvantage to provide the charging bin with a plurality of adjustabledischarge openings which, if desired, may be completely disconnected orshut oil. In smaller hearth furnaces where the charge is fed at a singlepoint, insteadof the above arrangement, a stationary charging devicemounted on the inner or outer side of the hood may be employed, which isprovided with discharge discs and which is completely encased. In allcases the charge material from the charging plate or disc may beconducted through pipes or slide surfaces to the charge material on thehearth whereby the possibility of dust formation is still furtherreduced.

In the case when ores causing very high temperatures during theoperation of the furnace are employed, it is of advantage to avoidthrowing the fresh charge material on the charge present on the hearthand to embed the fresh charge into the old and partly treated material.To this purpose the charging of the fresh material is provided in theregion of the working door so that the freshly added charge falls inproximity of the back wall of the hearth on top of the broken up chargewhere the removal of the slag is approximately finished. By returningthe portions of the charge which during the breaking up operation havebeen displaced towards the exterior rim of the hearth and into theirformer position, this returned charge will cover the freshly added andcold charge matei-ial. Preferably, this procedure is carried outmanually by the smelter in charge of the hearth with a scraper, in thesame manner as in the conventional manually operated simple lead,hearths. Thereby the intermediate and the lower portions. of the chargeare cooled and excessive temperatures within the charge are 7 avoided.As those skilled in the art know, m n

operating temperatures have the disadvantage that not only thevolatilization of lead but also the formation of slag are greatlyincreased and the output of metal is considerably lowered. In rotaryhearth furnaces generally, water cooling is provided for the back wallof the hearth for the purpose of reducing the working temperatures andfor carrying away heat from thecharge. This cooling is furthersupplemented by the cooling effect of the air entering from the outsideunderneath the hood. According to the present invention, the intimatecontact of this air with the charge and thereby emcient coolingconditions are obtained by the special arrangement and construction ofthe hood and of the -gas outlets. Since in the prior rotary hearthfurnaces the major portion of the air is introduced through the workingdoor into the hood, in the rotary hearth furnace embodying the inventionand employing only one working door, this door is located at one sideand the gas outlet at the other side of the furnace ring so that the gasoutlet is provided in the portion of the hood directly opposite to theworking door. Thus the air entering through the working door isuniformly distributed to both sides and a uniform gas stream isdeveloped above each half of the hearth between each working doorand'gas outlet without having obstacles in the path of these streams andwithout causing any eddy currents which facilitate the production offlying dust. The hood may be made large enough to considerably reducethe velocity of the gas stream within the hood.

Likewise, and in consideration of the same principles, in a hearthhaving a plurality of working doors, the outlets i'or the furnace gasesare preferably provided approximately half way between two successiveworking doors. This makes it possibleto obtain the most favorableconditions for the streaming of the gases and for the dissipation of theheat. It has been ascertained that in this manner the formation offlying dust and particularly the volatilization of lead are stillfurther reduced. Increasing the size of the hood has the furtheradvantage that a great part of the flying dust possibly formed'isseparated from the gases within the hood and falls directly back on thecharge in the hearth. The separation of dust in hot condition above thehearth can be further promoted by special construction or shape of thehood. These conditions may be obtained, for example, by subdividing theinner space of the hood in the form of a cyclone separator or bychanging the direction or speed of the stream of waste gases within thehood. Thus, the space within the hood may be subdivided by means of aninclined annular wall. At suitable portions of the wall openings may beprovided for the passage of the gases from the lower space or portion ofthe hood into the upper space thereof and the gas outlet pipes areconnected to suitable portions of the upper space. The dust deposited inthe upper space of the hood is returned to the hearth by means of smallslots in said inclined annular wall which are preferably arranged in theproximity of the cooled wall of the hearth. The same results, to wit:returning the dust to the hearth, may be *obtainecf by providing arelatively low hood in combination with high and very wide gas outletpipes, (Fig. 5) 28 above the hood. These gas outlet pipes act in themanner of a dust chamber so that part of the dust contained in the gasesis deposited therein and is returned to the charge on the hearth.

For the purpose of giving one skilled in the art a better understandingof the invention, a preferred embodiment will be described inconjunction with the accompanying drawings.

Referring more particularly to Fig. 1, an annular hearth I is rotatablymounted by means of rails 2 on rollers 3. Hearth I is driven by means ofa motor (not shown) and a. reducing gear II through a pinion 39 andcrown wheel 4. Nozzles 5 are provided around the circumference of thehearth and are supplied with blasting air through a wind box 6 connectedto the hearth and rotating therewith. Wind box '6 is in turn suppliedwith air through a centrally located stationary pipe I locatedunderneath the wind box. Stationary air pipe is connected to a suitablesource of air such as blower, or the like having a conduit discharginginto pipe I through an opening 8. The air pipe is provided with a manhole 62, through which the inside of the pipe is accessible forinspection and cleaning.

Hearth I is centered and wind box 6 is connected to pipe I in asubstantially air tight manner through sliding surfaces 9 and packingseal III. A support IB bearing the rotatably mounted rods I! and I8 issecured to the upper end of the stationary air supply pipe I in such amanner that its axis of rotation is approximately in the same verticalplane as the rabbles I2 and I3 of a rabbling apparatus I4. This willcause rods I'I, I8, etc. to successively engage and disengagecorresponding nozzles in the hearth and will close the nozzles duringthe time when they pass by the rabbles and if necessary also during thetime the nozzles are in the region or in a part of the region of theworking door. Further similar devices for intermittently shutting offthe nozzles may be provided at other portions of the furnace, ifnecessary.

The operation of these devices for controlling the nozzles may be morecompletely understood by referring to Fig. 6 illustrating a topelevational view of the device for closing and cleaning the nozzles anda sectional view of the hearth wall taken in the plane of the nozzles. A

. vertical shaft I5 around which rod supporting member I6 bearing rodsI1, I8 and I9 is mounted, is rotatably supported in a bracket 20, whichis rigidly connected to pipe I. Rod I8 is shown being within conicallyshaped nozzle 2I, while.

rod ll just begins to enter nozzle 22 and rod I9 leaves nozzle 23. Therotation of hearth I will cause the nozzles H to engage and displace rodI8 until the next rod I! completely engages nozzle 22. Thus. therotation of the rods and of their support is caused and is directlycontrolled by the rotation of the hearth. This arrangement has the greatadvantage of providing positive control and closing of the nozzles andat the same time deposits and incrustations within the nozzles areautomatically removed and returned to the hearth and cannot drop intothe wind box. In other words, the device provides positive control andautomatic cleaning of the nozzles in a simple and foolproof manner.

Hearth I of the furnace is provided with a water cooled back wall 24'which rotates with the hearth. A gas tight joint between back wall 24and the inner wall 25 of stationary hood 26 is provided by means of awater or sand seal 21. The gases are removed from the hood through widegas outlet pipes 28 which are mounted in the cover of the hood betweentwo working doors.

In case more than two working doors are employed, the number of gasoutlet pipes is increased accordingly. In order to maintain the leadbath in the hearth at a substantially uniform level throughout theoperation of the rotary hearth furnace, a circular channel 48 isprovided around the hearth. The lead discharged from the hearth throughdischarge spout 63 flows into channel 48 and finally arrives into acollecting and casting receptacle 49. in admixture withcarbonaceousmaterials and in some cases with other additional reducingagents, is introduced in conventional manner onto an annular chargingplate 59 rotated with the hearth and is charged from this plate to thehearth. The outer edge of the hearth is provided with a working plate 64which facilitates the removal of slag from the broken up charge.

The relatively high and wide hood, such as 26 in Fig. 1, is particularlyadvantageous in smaller furnaces. In rotary hearth furnaces of largersize the hood may be constructed in the form illustrated in Figs. 2 and3. From the illustration of this modified embodiment of the inventionthe drive for rotating the hearth, the air conduits'and the device forcontrolling the nozzles have been omitted for the sake of sim- The oreplcity. The air entering through the workingdoors 51 and the furnacegases are conducted in a regulated stream over the hearth and rise intothe upper portion of the hood through openings 58. The upper portion ofthe hood is defined by plates 29, 30, 3| and 32. Openings 58 are locatedapproximately above the middle of the hearth portion between the twoworking doors; whereas the gas outlets 28 are located above the workingdoors. In this form of the hood the gases not only provide good coolingof the charge on the hood but at the same time due to the change in thedirection of their streaming and in their velocity and due to theirreflection, most of the dust carried by the gases is precipitated. Theprecipitated dust slides downwards along inclined surfaces 28 and 30 andarrives at a gap 33 through which it drops back on top of the materialon the hearth. Gas outlet pipes 28 are extended into the upper space ofthe hood so that a streaming of the gases is produced similar to thatprevailing in cyclone separators whereby the precipitation of dust isstill further improved. The hearth is provided with a rabbling apparatusi4 having a pair of rabbles I2 and i 3.

In the modified embodiment of the invention shown in Figs. 4 and 5 arelatively low hood 26 is employed, which will cause substantiallyhigher velocities of the gases above the hearth. The precipitation ofdust is performed in the very high and wide gas outlet pipes 28 which inaddition may be constructed in the manner of a cyclone separator.Preferably, the device'conveying the charge to the hearth is provided inclose proximity to the hearth. A low cooling sleeve 24 carries acharging ring 34 on which the ore is deposited by the discharge openings35 of the charging bin. Discharge openings 35 and the conduitsassociated therewith are of a quadratic cross section and extend throughcover 52 of the dust collecting hood approximately to the charging ring.Looking in the direction of hearth rotation, in the back wall of eachcharging outlet there is an opening cut out, the free width of which maybe adjusted by means of a slide 36. Slide-36 may be opened to a smalleror, greater extent, or may be completely closed by means of a threadedspindle 6|. The charge material is thrown on the hearth by means of ascraper60 shortly after it has been deposited on the charging ring. Thewind box and the nozzle cleaning device are accessible through the airpipe which to arrange the charging apparatus and the conduits for thecooling water in this space. In some cases it is of advantage to providea sheet metal ring 31 around the outer wall of the hood whereby thestreaming of air along the outer surface of the hood is regulated andthe cooling of the outer wall is ftu'ther promoted. A rabbling apparatusl4 having rabbles l2 and His provided at one side of the hearth.

Fig. 7 is a detail view of the rabbling apparatus l4 for actuatingrabbles l2 and I3. .A pair of discs or ecce'ntrics 40 and 4| isprovided, supporting rabbles l2 and I3 displaced by about 180 withrespect to each other. Discs 40 and 4| are rotated by means of a motor42, through a reduc-' ing gear 43, a shaft 44 and drive wheels 45 and 46of which one, wheel 46, may be disengaged by means of a clutch 41. Thespeed of rotation of discs 46 and 4| and therotation' of the hearth areso adjusted that the strokes of the rabbles overlap and that all of thesurface of the hearth and the charge therein are uniformly subjectedtothe rabbling treatment.

The rotary hearth furnace illustrated in Figs. 8 and 9 is similar inconstruction to the ones shown in Figs. 1 to '7 and similar referencecharacters are used to denote corresponding parts.

This hearth is provided with a single charging apparatus 66. The ore ischarged from a bunker 66 into a tube 68 through the intermediary of adischarge plate 61 arranged within a closed housing 66. Tube 68 iscarried through hood 26 of the hearth which extends downwardly almost tothe hearth. This hearth furnace is provided with a single working door69. Opposite to the working door 691s located draw-off ill for thehearth gases. Thus, the air introduced underneath the hood throughworking door 69 is divided into two branches streaming right and lefttowards drawoff 10 above the twmhalves of the hearth. These .twobranches of streaming air take up the hearth gases and are again unitedafter leaving the hearth.

We claim: 1. A rotary hearth furnace for the production of lead fromsulphide ores comprising an annular rotary hearth, a stationary hoodabove said" hearth, said hood extending close to the outer rim of thehearth, a windbox in the center of the hearth, a plurality of nozzlesoperatively assosaid hood to said hearth, a plurality of nozzlesassociated with the inner rim of said hearth and in Communication withsaid windbox, and means windbox in the center of the hearth, a pluralityoi."

nozzles associated with the inner rim of said hearth at spaced intervalsof the circumference thereof in communication with said windbox, meansfor maintaining said hearth in continuous rotation, and means forintermittently shutting off the passage of air through said nozzlesduring the time they are in proximity of said rabble arms.

4. A rotary hearth furnace for the production of lead from sulphide orescomprising an annular rotary hearth, a stationary hood above saidhearth, said hood extending close to the outer rim of the hearth, atleast one rabble arm extending through said hood to said hearth, awindbox in the center of the hearth, a plurality of nozzles associatedwith the inner rim of said hearth at spaced intervals of thecircumference thereof in communication with said windbox, a rotatablemember in said windbox, and a plurality of rods radially andequidistantlally mounted on said member, said rods being adapted toengage said nozzles during rotation of said hearth and to impart rotarydisplacement to said member for intermittently shutting off and to cleansaid nozzles.

5. A rotary hearth furnace for the production of lead from sulphide orescomprising an annular rotary hearth, a. stationary hood above saidhearth, said hood extending close to the outer rim of the hearth, atleast one rabble arm extending through said hood to said hearth, awindbox in the center of the hearth, a plurality of nozzles associatedwith the inner rim of said hearth at spaced intervals of thecircumference thereof in communication with said windbox, means formaintaining said hearth in continuous rotation, a rotatable member insaid windbox, a'

shaft for said member located substantially in the plane of said rabblearm, and a plurality of rods radially and equidistantially mounted onsaid member, said rods being adapted to be engaged by said nozzlesduring rotation of said hearth and to impart rotary displacement to saidmembers for successively and intermittently shutting off and forcleaning the nozzles within the. region of said rabble arm.

6. A rotary hearth for the production of lead from sulphide orescomprising an annular rotary hearth, a stationary hood above said hearthextending close to the outer-rim thereof, a windbox in the center ofsaid hearth, a windpipe communicating with said windbox and coaxialtherewith, a plurality of tapering nozzles associated with the inner rimof said hearth at spaced intervals of the circumference thereof incommunication with said windbox, rabble arms extending through said hoodto said hearth, a rotatable member in said windbox, a vertical shaft forsaid member mounted on said windpipe substantially in the plane of saidrabble arms, and a plurality of rods radially and equidistantiallymounted on said member, said rods being adapted to be engaged by saidnozzles during rotation of said hearth whereby a rotary displacementwill be imparted to said members and the nozzles within the region ofsaid rabble arms will be positively and automatically controlled andcleaned.

7. A rotary hearth furnace for the production of lead from sulphide orescomprising an annular rotary hearth, -a stationary hood above saidhearth extending close to the outer rim thereof, a windbox in the centerof the hearth, a plurality of nozzles associated with the inner rim ofsaid hearth at spaced intervals of the circumference thereof incommunication with said windbox, at least one rabbling device located inproximity of said hood, a pair of rabble arms associated with saidrabbling device and extending through an opening in said hood to saidhearth in close proximity to each other, a roller underneath saidopening for each of said rabble arms for slidably supporting the same,means including a rotatable disc for each of said arms forreciprocatingly and alternately actuating said arms, means formaintaining said hearth in continuous rotation, and means forintermittently shutting off the passage of air through said nozzlesduring the time they are in proximity to said rabble arms.

8. A rotary hearth furnace for the production of lead from sulphide orescomprising an annular rotary hearth, a stationary hood above said hearthextending close to the outer rim thereof, a windbox in the center of thehearth, a plurality of nozzles associated with the inner rim of saidhearth at spaced intervals of the circumference thereof in communicationwith said windbox, at least one rabbling device located in proximity ofsaidhood. a shaft associated with said device, a pair of eccentricsdriven by said shaft, a rabble arm for each of said eccentrics havingone of its ends connected to said eccentric and having its other endextending through an opening in said hood to said hearth, said rabblearms being located in close proximity to each other, a roller underneathsaid opening for slideably supporting said arms, means for driving saidshaft for reciprocatingly and alternately actuating said arms, means formaintaining said hearth in continuous rotation, and means forintermittently shutting -oif the passage of air through said nozzlesduring the time they are in proximity to said abble arms. F

9.v A rotary hearth furnace for the production of lead from sulphideores comprising an annular rotary hearth, astationary hood above saidhearth extending close to the outer rim thereof, a windbox in the centerof ,the hearth, a plurality of nozzles associated with the inner rim ofsaid hearth at spaced intervals of the circumference thereof incommunication with said windbox, a

- rabbling device having at least two rabble arms extending through saidhood to said hearth in close proximity to each other, means forreciprocatingly and alternately actuating said rabble arms, means formaintaining said hearth in continuous rotation, and means forintermittently shutting off the passage of air through said nozzlesduring the time they are in proximity of said rabble arms.

10. A rotary hearth furnace for the production of lead from sulphideores comprising an annular rotary hearth, a stationary h od above saidhearth, said hood extending clo e to the outer rim of the hearth, awindbox in the center of the hearth, at least one rabble arm extendingthrough said hood to said hearth, a plurality of nozzles associated withthe inner rim of said hearth and in communication with said windbox,

means for intermittently and successively shutting off the passage ofair through said nozzles.

means for charging ore to said hearth, means for discharging leadtherefrom, and at least one working door for handling the charge on thehearth.

11. A rotary hearth furnace for carrying out a continuous roastingreaction process comprising an annular hearth of substantiallysemi-circular cross section, a stationary hood above the hearth, arabbling device including a pair of rabbling arms alternately actuatedby a pair of eccentrics and extending through the hood to the hearth, awindbox in the center of the hearth, a plurality of tapered nozzlesassociated with the inner rim circular cross section, a stationary hoodabove the hearth, a rabbling device includinga pair of rabbling armsalternately actuated by a pair of eccentrics and extending through thehood to the hearth, a windbox in the center of the hearth, a pluralityof tapered nozzles associated with the inner rim of said hearth and incommunication with said windbox, means for controlling the passage ofair through said nozzles in accordance with the rotation of said hearth,an annular charging plate arranged at a slight height above the hearthand rotatingwith said hearth, a charging hopper for depositing chargematerial on said charging plate, and a scraper for stripping oif saidmaterial from said plate and to deposit said material on the hearth.

13. A rotary hearth furnace for the production of lead from sulphideores comprising an annular rotary hearth, a stationary hood above saidhearth extending close to the outer rim of the hearth, awindbox in thecenter of the hearth, a plurality of nozzles associated with the innerrim of said hearth in communication with said windbox, rabble armsextending through said hood to said hearth, means .for intermittentlyshutting ofl air through said nozzles during the time they are inproximity of said rabble arms, partition walls separating said hood intoan upper and-a lower portion, working doors in said lower portion forhandling the charge on the hearth, gas outlet pipes extending into saidupper portion, and openings in said partition walls for permitting thepassage of gases from said lower into said upper portion of the hood.

14. A rotary hearth furnace for the-production of lead from sulphideores comprising an annuiarrotary hearth, a stationaryhood above saidhearth extending close to the outer rim of the hearth, a windbox in thecenter of the hearth, a plurality of nozzles associated with the innerrim of said hearthin communication with said windbox, rabble armsextending through said hood to said hearth, means for intermittentlyshutting of! air through said nozzles during the time they are inproximity of said rabble arms,

inclined partition walls separating said hood into an upper and a lowerportion, working doors in said lower portion for handling the chargeonthe hearth, gas outlet pipes extending into said upper portion,openings in said partition walls to establish communication between saidupper and lower portions, and a gap in the lowermost portion of saidinclined partition walls for returning precipitated dust to the hearth.

15. A rotary hearth furnace for the produc-- tion of lead from sulphideores comprising an annular rotary hearth, a stationary hood above saidhearth extending close to the outer rim of the hearth, a windbox in thecenter of the hearth, a plurality of nozzles associated with the innerrim of said hearth in communication with said windbox, rabble armsextending through said hood to said hearth, means for ining doors toestablish communication between said upper and lower portions, and a gapin the lowermost portion of said inclined partition walls for returningprecipitated dust to the hearth; v

16. A rotary hearth furnace for the production of lead from sulphideores comprising an annular rotary hearth, a relatively low stationaryhood above said hearth adapted to cause high' gas velocities, a windboxin the center of the hearth, a plurality of nozzles associated withworking doors, and adapted to cause the precipitation of dust therein.

17. A rotary hearth furnace for the produc? tion of lead from sulphideores comprising an annularrotary hearth, a relatively low stationaryhood above said hearth adapted to cause high gas velocities, a windboxin the center of the hearth, a plurality of nozzles associated with theinner rim of said hearth in communication with said windbox, rabble armsextending through said 'hood to said hearth, means for intermittentlyshutting off air through said noz-- zles during the time they are inproximity of said rabble arms, working doors in said hood for handlingthe charge on the hearth, and high gas outlet pipesprotruding into thetop portion of said hood, said pipes being constructed in the form of acyclone separator and being adapted to precipitate dust therein.

18. A rotary hearth furnace for the production of lead from sulphideores comprising an annular rotary hearth, a relatively low stationaryhood above said hearth adapted to cause high gas velocities, a windboxin the center of the hearth, a plurality of nozzles associated with theinner rim of said hearth in communication with said windbox, rabble armsextending through said hood to said hearth, means for in-- termittentlyshutting 01f air through said nozzles during the time they are inproximity of said rabble arms, working doors in said hood for handlingthe charge on the hearth, high gas outlet pipes protruding into the topportion of said hood located equidistantially from said working doorsand adapted to cause the deposition of dust therein, a charging hopperfor feeding ore to said. hearth, a ledge rotating with said hearth andarranged at a slight height above the hearth upon which-the ore isdeposited from said hopper, and an adjustable scraper for scraping orefrom said ledge on to said hearth.

